Field of the Invention
The present invention relates to an electromagnetic wave absorber having a hollow structure, and to an electromagnetic wave anechoic room using the absorber.
Description of the Prior Art
As an electromagnetic wave absorber used in an electromagnetic wave anechoic room, a solid absorber of a pyramid (quadrangular pyramid) or wedge shape conventionally has been often employed. Also for weight and cost reduction, a hollow absorber of a pyramid or wedge shape conventionally has been often employed.
Generally, a hollow electromagnetic wave absorber is composed of plate-like electromagnetic wave absorption members containing a resistance material as carbon etc. or plate-like electromagnetic wave absorption members including a resistance layer on a surface thereof. A foamed styrol board containing carbon inside, a corrugated board structure consisting of mixed papers or a plastic board containing carbon inside, and a foamed styrol board provided with a resistance layer on a surface thereof etc. are examples of the plate-like electromagnetic wave absorption member.
When an electric field of an incident electromagnetic wave and the plate-like electromagnetic wave absorption member are parallel, both absorption and surface-reflection are large in electromagnetic wave absorption characteristics of the plate-like electromagnetic wave absorption member, and when the electric field of the incident electromagnetic wave and the plate-like electromagnetic wave absorption member are vertical, both absorption and surface-reflection are small.
FIG. 10A, FIG. 10B and FIG. 10C show a prior art of an electromagnetic wave absorber having a hollow pyramid shape and a combination of plate-like electromagnetic wave absorption members. FIG. 10A is a plan view as seen from an incident direction of an electromagnetic wave, FIG. 10B is a side view, and FIG. 10C is a XC-XC line cross sectional view of FIG. 10B. In the cross sectional view shown FIG. 10C of the prior art, the electromagnetic wave absorption member 1 parallel to the electric field of the incident electromagnetic wave has large absorption and large surface-reflection, and the electromagnetic wave absorption member 2 vertical to the electric field of the incident electromagnetic wave has small absorption and small surface-reflection. Namely, when the incident electromagnetic wave is high frequency of short wave length, in an area A of FIG. 10A as seen from the incident direction of electromagnetic wave, the absorption is large because the electric field of the incident electromagnetic wave and the electromagnetic wave absorption member are parallel. On the other hand, in an area B of FIG. 10A the absorption is small because the electric field of the incident electromagnetic wave and the electromagnetic wave absorption member are vertical, and large reflection is caused by the area B. When a plurality of electromagnetic wave absorbers 3 of FIG. 10A, FIG. 10B and FIG. 10C are arranged as shown in FIG. 11A and FIG. 11B, the surface-reflection is large in the area A, and also multipath reflection is increased.
As a result of the above, it is apparent that the conventional hollow electromagnetic wave absorber has a problem that the electromagnetic wave absorption characteristics are low in high frequency.
Conventional electromagnetic wave absorbers for constituting electromagnetic wave anechoic rooms are disclosed in Japanese Utility Model Application Laid-Open No. 1-171096, Japanese Patent No. 4988060, and Japanese Patent No. 4420253.
In case of disposing a plurality of electromagnetic wave absorbers on a side wall surface (or side door), Japanese Utility Model Application Laid-Open No. 1-171096 as shown in FIG. 12A and FIG. 12B discloses a configuration in which electromagnetic wave absorbers 5 consisting of parallel arrangements of chevron-shaped electromagnetic wave absorption members are disposed on the side wall surface 7 so as to be inclined about 45° to a line of intersection between a floor surface 6 and the side wall surface 7 (or side door).
In this case, whether the electric field direction of the incident electromagnetic wave is horizontal or vertical (horizontally-polarized wave or vertically-polarized wave), it seems to be expected effects of improving electromagnetic wave absorption characteristics in a high frequency range, because the electric field direction is not vertical to the outer surfaces of the electromagnetic wave absorbers 5. However, as shown in FIG. 12A and FIG. 12B, many jagged parts disable to arrange the electromagnetic wave absorbers are existing around end portions of wall surfaces and near areas of an opening as a door or the like. Because reflections of the jagged parts degrade the absorption characteristics of an electromagnetic wave anechoic room, there is a problem that must fill the jagged parts by disposing electromagnetic wave absorbers specially processed.
Japanese Patent No. 4988060 discloses an electromagnetic wave absorber unit which integrally has a plurality of pyramid shape electromagnetic wave absorbers inclined at 45°. In this case, the similar problem exists because the electromagnetic wave absorber units form a jagged outline shape when the units are arranged side by side. For instance, parts disable to arrange the electromagnetic wave absorbers are existing around end portions of wall surfaces and near areas of an opening as a door or the like of an electromagnetic wave anechoic room, therefore there is a problem that must fill the parts by disposing electromagnetic wave absorbers specially processed.
As shown in FIG. 13A and FIG. 13B, Japanese Patent No. 4420253 discloses a electromagnetic wave absorber 10 which has side face forming parts 12 forming side faces of the hollow pyramid 11 respectively, and extending parts 13 each of which extended to protrude from one ridgeline of each part 12 in the same plane from the part 12, wherein the extending part 13 is present only one for one ridgeline. A magnetic loss body 15 of plat-like ferrites (ferrite tiles) etc. is arranged at a bottom side of the electromagnetic wave absorber 10. Japanese Patent No. 4420253 alleges that favorable electromagnetic wave absorption characteristics are obtained in a wide range of from low frequency to high frequency. However, because the side face forming part 12 and the extending part 13 which extends in the same plane of the part 12 are parallel or vertical to the electric field direction of the incident electromagnetic wave, decrease of electromagnetic wave absorption characteristics caused by areas vertical to the electric field direction of the incident electromagnetic wave are not ignorable.